Sputtering describes a number of physical techniques commonly used in, for example, the semiconductor industry for the deposition of thin films of various metals such as aluminum, aluminum alloys, refractory metal silicides, gold, copper, titanium-tungsten, tungsten, molybdenum, tantalum and less commonly silicon dioxide and silicon on an item (a substrate), for example a wafer or glass plate being processed. In general, the techniques involve producing a gas plasma of ionized inert gas "particles" (atoms or molecules) by using an electrical field in an evacuated chamber. The ionized particles are then directed toward a "target" and collide with it. As a result of the collisions, free atoms or groups of ionized atoms of the target material are ejected from the surface of the target, essentially converting the target material to free atoms or molecules. Most of the free atoms which escape the target surface in the direction of the substrate, strike the substrate without intervening collision, and form (deposit) a thin film on the surface of the object (e.g. wafer, substrate) being processed, which is located a relatively short distance from the target.
One common sputtering technique is magnetron sputtering. When processing wafers using magnetron sputtering, a magnetic field is used to concentrate sputtering action in the region of the magnetic field so that target sputtering occurs at a higher rate and at a lower process pressure. The target itself is electrically biased with respect to the wafer and chamber, and functions as a cathode. Objectives in engineering the cathode and its associated magnetic field source include uniform erosion of the target and uniform deposition of pure target material on the wafer being processed. During sputtering, if magnets generating a magnetic field are stationary at a location, then continuous sputtering quickly consumes the sputtering target thickness at that location and generates hot spots at the locations of sputtering. Therefore, the magnets are usually continuously moved over the back of the target during its sputtering. Nonetheless, non-uniform wear patterns persist. To avoid contaminating the process, sputtering is stopped before the non-uniform sputtering wear pattern has consumed the full thickness of the target material at any point. If any point on the target plate behind the target were to be reached, sputtering of the target backing plate material (often copper) would occur, contaminating the vacuum chamber and the water being processed with the target backing material (e.g., copper). Because of the non-uniform pattern of target utilization, conventional practice is to stop the sputtering while a large percentage of the target still remains.
As the target erodes, the distance between the target surface (which is eroding away) and the substrate being sputtered is slowly increasing. The change in the distance between the target surface and the substrate being sputtered creates a change in the qualities of the sputtered material deposited and its uniformity. When large areas such as glass plates are being deposited, variations in the thickness of deposited sputtered material are measurable and may be unacceptable if the change in the target thickness detrimentally affects the deposition of the target material on the substrate being deposited.
Considerable energy is used in generating the gas plasma and creating ion streams impacting on the cathode. This energy must be dissipated to avoid melting or nearly melting the structures and components involved. Common techniques used for cooling sputtering targets are shown in FIGS. 1 and 2. One technique, as used in many prior art sputtering devices, passes water or other cooling liquid through a fixed internal passage of the sputtering target as shown in FIG. 1, a first cooling liquid passageway such as a hose 65 supplies water or other cooling liquid to a target backing plate 63 where it passes through cavities or passages of the backing plate and out a second hose 66. The target 64 is thereby quickly cooled. To complete the picture of FIG. 1, the sputtering chamber 60 includes an object (substrate) support structure 62 on which rests the substrate to be deposited 61. In this configuration the sputtering target is completely immersed in the process environment. A water-to-vacuum seal is often needed to prevent the water or other cooling liquid from leaking out of its passages. A magnetron sputtering cathode as described by Carlos dos Santos Pereiro Ribeiro in his U.S. Pat. No. 4,826,584 is typical of the prior art showing cooling line attachments hidden behind the sputtering target and attached to the back of the target to pass liquid through structures adjacent to the target. While a magnet is not shown in FIG. 1, commonly these devices have stationary or rotating magnets to assist in directing the ion flow and controlling the primary sputtering location.
Another technique which is commonly used which has eliminated the vacuum-to-water seal problem is shown in FIG. 2. A processing chamber 50 includes a sputtering table 56 supporting a substrate 55 to be sputter coated in close proximity to a target 54. The sputtering chamber 50 includes a circumferential top flange on which a target assembly 52 rests. The target assembly 52, consisting of a target backing plate 53 and the target 54, completely covers the flange of the processing chamber 50. A seal is made between the processing chamber 50 and ambient air outside the flange. A cooling chamber 51 encloses the top of the target assembly 52. A stationary magnet or a moveable magnet 57 is located closely adjacent to the back of the target backing plate 53. A magnet sweep mechanism (not shown) causes the movable magnet 57 to move in a magnet sweep zone as shown by the dashed lines 58. The moveable magnet 57 and portions of the magnet sweep mechanism (not shown), in this configuration, are immersed in cooling liquid which generally fills the cooling chamber 51 and is circulated through the chamber behind the target to ensure cooling of the target.
In these configurations the target backing plate 53 is subjected to a strong vacuum pressure on the processing side (less than 1 torr) with a positive pressure of as much as several atmospheres on the cooling side. The actual pressure on the cooling side depends on the weight of the coolant in the cooling chamber and on the static and dynamic pressures of the coolant in the cooling chamber as enough coolant to maintain acceptable temperatures on the sputtering target is moved through the system. To avoid short-circuiting the flow of coolant through the chamber (from the inlet immediately by the shortest path to the discharge), often distribution manifolds or flow directing restrictions are placed in the path of the coolant:to minimize short-circuiting and maximize cooling.
To obtain maximum thermal conduction between the cooling liquid and the target backing plate, it is necessary that the flow regime in the cooling chamber 51 be such that any fluid boundary layer formed at and near the back of the target backing plate 53 be minimized or eliminated. Therefore, laminar cooling flow is not sufficient, the flow must be in the turbulent range to maximize heat transfer between the fluid target and the fluid. Higher fluid pressures are needed to generate the fluid velocities required for turbulent flow resulting in higher pressures in the cooling chamber 51.
For small sputtering targets, the target backing plate and target can be built to be quite massive to resist deflections due to the differential pressures between the vacuum in the processing chamber 50 and the ambient air pressure plus fluid pressure in the cooling chamber 51. When the size of the target and target backing plate become large because the area to be coated is quite large, as might be done for a flat glass panel to be used in a flat panel display of a computer or television screen, the thickness of the target and target backing plate must be substantially increased to avoid unacceptable deflections. When a magnetron is used, sputtering is most effective when the magnets are just behind the surface of the sputtering target. Increasing the distance between the surface of the sputtering target and any magnets used for magnetron sputtering behind the target (by increasing target thickness) substantially decreases the effect of the magnets on sputtering, or conversely, much more powerful magnets need to be used in order to be sure that the magnet field is effective through the thickness of the thicker target and its backing plate. The target assembly, in its present target size of 510 mm.times.620 mm, or 570.times.720, however, does not require the top chamber to be under vacuum for the system to operate properly because the deflecting loads due to vacuum from the process-side chamber are small and the coolant loads are all internal to the target assembly.
The deflection of the target and target backing plate under the differential pressure between the processing chamber and the cooling chamber causes the target and target backing plate to bow substantially. Many targets are attached to their target backing plates using a relatively ineffective soldering or brazing technique. The bowing of the target backing plate and target creates an enormous stress in the solder or brazing material, or in the target material if it is softer, such that the probability of de-lamination or separation of the target from the target backing plate is greatly increased. In instances where solder or low temperature brazing has been used, a separation between the target and target backing plate at one point acts as a nucleus for a propagating defect. Once a pinhole surface defect forms, a hot spot develops progressively melting the solder and brazing compounds located there. When sufficient melting and/or separation has occurred, the target will actually drop off the target backing plate, ruining the process and requiting complete replacement of the target, if not a complete cleaning of the process chamber.
Methods that have been used in the past to attempt to overcome these difficulties include high pressure--high temperature diffusion bonding, explosion bonding, friction welding, or roll bonding of the target to its backing plate. In these processes there is a large non-uniform thermal or mechanical gradient to which the target and target backing plate are subjected. The microstructure of the various pieces is affected by the stress induced by thermal gradients or mechanical deflections and the dimensions of the pieces change. Subsequent processing (machining and or thermal stress reduction techniques) must often be used to arrive at a target-target backing plate assembly that is dimensionally stable without warpage under the thermal cycling of sputtering.
The disadvantages of the existing sputtering target systems as described above continue to inhibit the wide use of sputtering as an efficient and cost-effective means for applying surface coatings.